A human placental DNA fragment containing a sequence homologous to the transforming gene (v-mos) of Moloney murine sarcoma virus was molecularly cloned. The DNA sequence of the homologous region of human DNA (termed humos) was compared to that of the mouse cellular homolog of v-mos (termed mumos) as reported by Van Beveren, et al. (Cell 27: 97-108, 1981). The humos gene contained an open reading frame of 346 codons that was aligned with the equivalent mumos DNA sequence. The aligned coding sequences were 77% homologous and terminated at equivalent opal codons. The humos open reading frame initiated at an ATG found internally in the mumos coding sequence. The polypeptides predicted from the DNA sequence to be encoded by humos and mumos were also found to be extensively homologous and 253 of 337 amino acids of the humos gene product were in common with those of mumos. In addition, near the middle of the polypeptide chains, four regions ranging from 19-26 consecutive amino acids were conserved. However, we have not been able to transform mouse cells with transfected humos DNA fragments or with hybrid DNA recombinants containing humos and retroviral long terminal repeat (LTR) sequences.